The present invention relates to a scanning optical system used for an image formation device such as a laser printer.
In a scanning optical system, a beam of light emitted by a light source such as a laser diode is dynamically deflected (scanned) by a deflecting system such as a polygon mirror, and the beam scanned by the deflecting system is focused by a scan lens group such as an fθ lens to form a spot on an image formation surface of a photosensitive drum, etc. The spot which is scanned in a main scanning direction repetitively on the image formation surface successively forms a scan line on the image formation surface. Therefore, with the scanning optical system, by moving the image formation surface relative to the scanning optical scanning system at a constant speed in an auxiliary scanning direction (orthogonal to the main scanning direction) and on/off modulating the laser beam in sync with the scanning operation, a two-dimensional electrostatic latent image is formed on the image formation surface.
The scan lens group of the scanning optical system does not have a function of correcting chromatic aberration since the scan lens group is generally designed assuming the use of a beam of a single wavelength. Therefore, if the oscillation wavelength of the laser diode changes (due to individual differences, changes in the temperature and the output level, etc.), the length of each scan line formed on the image formation surface fluctuates due to chromatic aberration of magnification of the scan lens group, by which the precision of image formation is deteriorated.
In order to eliminate the effect of the chromatic aberration of magnification of the scan lens group, some configurations, employing a diffractive level difference structure (like a Fresnel lens) formed on a lens surface of a refractive lens of the scan lens group, have been disclosed in Japanese Patent Provisional Publication No.HEI10-197820 (hereinafter referred to as a “document #1”), Japanese Patent Provisional Publication No.HEI11-95145 (hereinafter referred to as a “document #2”), and Japanese Patent Provisional Publication No.2001-125025 (hereinafter referred to as a “document #3”).
An example of a manufacturing method of a long scan lens having such a diffractive level difference structure, employing injection molding of resin, has been described in the document #3. According to the document #3, the long scan lens having the diffractive level difference structure is manufactured by injection molding of resin by use of a mold having a gate 31 (for injecting the resin into the mold) at a central position in the main scanning direction so that there will be no blind spot for the flow of the resin injected into the mold (i.e. no part inside the mold where the flowing resin can not reach).
However, even if the gate for injecting the resin into the mold is provided at the central position of the mold as described in the document #3, when a long lens having a lens surface with the diffractive level difference structure is manufactured by the injection molding of resin, deformation (e.g., loss of shape, getting out of shape) of the diffractive level difference structure tends to occur. Specifically, as the resin inside the mold cools down and contracts, stress concentration occurs to each edge of the diffractive level difference structure. When the lens is removed from the mold and the stress is released, each edge of the diffractive level difference structure is deformed (i.e., loses its original shape). Since such stress concentration is caused by contraction of the resin toward the center of the mold, the stress concentration and the edge deformation (caused by the stress concentration) become more and more significant as the distance from the central position (the position of the optical axis) increases.
Due to such deformation of the edges, the diffractive level difference structure previously formed on the inner surface of the mold can not be transcribed correctly onto the lens surface, by which diffraction efficiency of the lens is deteriorated. Further, since the number (density) of annular zones of the diffractive level difference structure increases as the distance from the center of the lens increases (that is, the peripheral area of the lens has a larger number of edges than the central area of the lens), the effect of the edge deformation (the effect of the incorrect transcription of the diffractive level difference structure) on the optical performance of the lens is even more enhanced in the peripheral area of the lens, by which a drop in peripheral brightness is caused.